1. Field of Invention
This invention pertains to the calibration of magnetic tape drives.
2. Related Art and Other Considerations
In magnetic tape drives, magnetic tape is transported past a head unit whereon at least one, and usually both, of a write head and a read head are mounted. As the tape is transported past the head unit, the heads are employed to transduce information with respect to the tape. In a recording mode, the write head records tracks on the tape. Conversely, in a read or reproduction mode, tracks previously recorded on the tape are read.
Two types of magnetic tape drives are serpentine tape drives and helical scan tape drives. In serpentine tape drives, elongated tracks are recorded parallel to the direction of tape transport, e.g., along the major dimension of the tape, typically from a first end of the tape to a second end of the tape. In helical scan tape drives, the head unit is mounted on a rotating drum around which the tape is partially wrapped at a predetermined angle. In view of the geometry, helical tracks are stripes are recorded and read by the helical scan tape drive.
In magnetic tape drives, the tape is typically housed in a cartridge. In some tape drives, such as the helical scan drives, a portion of the tape is extracted from the cartridge into a tape path for operative encounter with the head unit. In other drives, upon opening of a cartridge lid or the like, a tape path is formed in which the head unit operatively encounters the tape. In either case, the tape path of the tape drive typically includes one or more tape guiding elements for properly guiding and aligning the tape as the tape is in transit toward the head unit.
A typical tape guide element 700 shown in FIG. 7A and FIG. 7B has the shape of a spool, Guide element 700 has upper and lower edge flanges 702, 704 and a rotating, barrel-like midsection 706, all concentrically positioned about a mounting pin 708. The top of mounting pin 708 is threaded for engagement with interior threads on upper edge flange 702. An expansion spring 710 is retained between a deck of the tape drive and an underside of lower edge flange 704.
Tape T is guided between upper flange 702 and lower flange 704 in the manner shown in FIG. 7A. To accommodate linear transport of tape T, guide midsection 706 rotates about bearing 712. Bearing 712 and edge flanges 702, 704 are movable along the vertical direction as depicted by arrow 720. However, the precise vertical position of guide member 700, and thus of tape T, is adjusted and retained by upper edge flange 702. By rotation of upper edge flange 702 about the threaded top end of mounting pin 708, the tape T can be set to a proper vertical height for feeding of tape T toward the head unit.
The vertical height for guide elements such as guide 700 of FIG. 7A must be calibrated for a tape drive, e.g., upon manufacture and for maintenance of the tape drive. To this end, tape drive manufacturers have long used "master alignment tapes" for the spatial adjustment of the tape guide elements of the tape path in order to locate the position of the tape relative to the head unit, e.g. the rotating drum or scanner in a helical scan recorder rotary head device. The master alignment tapes are prepared on a well-calibrated tape drive and then removed therefrom. The master alignment tape is then inserted into a tape drive to be adjusted, e.g., a just-manufactured tape drive, and read by the adjusted tape drive. From the readback signals of a master alignment tape acquired from the tested tape drive, the technician (or robot) makes the available spatial guide adjustments of the guide elements of the adjusted tape drive until a desired readback waveform is achieved (or nearly achieved).
The overall accuracy of the master alignment tape approach depends on how the master alignment tape is constructed and how it is used. In this regard, the spatial information about the relationship between the master alignment tape and the head unit is derived from the readback signal amplitudes. Any other factors causing fluctuations in readback signal amplitudes (i.e., inconsistent head-tape contact) may influence the results.
Tape drive manufacturers traditionally employ "single-scan" master alignment tapes. A "single-scan master alignment tape" for a helical scan drive has a series of tracks written by only one write head using substantially the native (+1X) linear tape speed and drum RPM. Single-scan master alignment tapes are read only by one read head of a tape drive, even if the tape drive has a plurality of read heads. The read head which actually reads the master alignment tape is herein called the "activated" read head. As the name implies, in the "single-scan" method each track written on the master alignment tape is scanned only once by the corresponding read head of the tape drive undergoing adjustment. To eliminate read head width effects and recorded track width effects from the readback signal amplitude, the pattern of read head paths is intentionally offset so that each read head path only partially overlaps each track recorded on the master alignment tape.
Assuming that the head-tape contact is perfectly consistent throughout the read head scan, variation in the peak readback signal amplitude (during the on-tape scan) is directly related to the variation in the spatial overlap between the read head path and the recorded track of the master alignment tape. Typically, the voltage waveforms from many on-tape read scans are averaged together to improve signal to noise ratio (SNR). Generally, the technician (or robot) adjusts the tape drive's tape guiding elements, e.g. as explained above, to minimize any variation in the peak readback signal amplitude. This necessarily implies that the read head path shape matches the recorded track shape of the master alignment tape.
If the read head-to-tape contact is not perfectly consistent throughout the scan, the single-scan method will result in a misadjustment of the tape drive in order to compensate for the poor head-tape contact. For example, if poor head-tape contact results in a loss of peak readback signal amplitude only near the start of scan region, the drive will be adjusted so that the read head path overlaps more of the recorded track in this area to compensate for the signal loss due to poor head-tape contact. Although the desired peak readback signal amplitude/shape is achieved, the drive's spatial alignment does not match the master alignment tape since the overlap is not consistent.
To solve the problems of the single-scan method, a dual-scan master calibration tape and method of usage therefor is disclosed in U.S. patent application Ser. No. 08/841,597, filed Apr. 30, 1997 by Magnusson, entitled "PREPARATION AND USAGE OF DUAL-SCAN MASTER CALIBRATION TAPE FOR MAGNETIC TAPE DRIVE", which is incorporated herein by reference. The dual-scan master calibration tape is generated by transporting the master calibration tape past a rotating drum whereon a write head is mounted and activating the write head only during every other rotation of the drum. In use, the dual-scan master calibration tape is transported past a read head in a manner whereby, for a track pre-recorded on the master calibration tape, the read head separately follows a first path over a bottom longitudinal edge portion of the track and a second path over a top longitudinal edge portion of the same track. A first path read signal is generated as the read head follows the first (bottom longitudinal edge) path over the track; a second path read signal is generated as the read head follows the second (top longitudinal edge) path over the track. Both the first path read signal and the second path read signal are used to determine a calibration indicia for the tape drive. In some embodiments, the dual master calibration tape is written at a non-native linear tape speed and is transported past the read head at a non-native linear tape speed.
In order to abate noise influences and improve the signal to noise ratio of the measurements, in actuality many first path read signals are averaged together and many second path read signals are averaged together before determining the calibration indicia for the tape drive. However, when making use of relative inexpensive instrumentation, an inconvenient number of signals are required for such averaging to provide near real-time feedback for adjustments.
What is needed therefore, and an object of the present invention, is a calibration method for a tape drive which is relatively expeditious and for which extensive signal averaging is not required.